The flux in any portion of the armature passes through p/2 c/r (cycles per revolution) or through (p/2 [(r/min)/60] Hz. The iron losses consist of the hysteresis loss, which equals Kb1.6fw watts, and the eddy current loss, which equals Ke(Bft)^2w watts.

K is the hysteresis constant of the iron used, Ke is a constant inversely proportional to the electrical resistance of the iron, B is the maximum flux density in lines per square inch, f is the frequency in hertz, w is the weight in pounds, and t is the thickness of the core laminations in inches.

The eddy loss is reduced by using iron with as high an electrical resistance as is feasible. Very high resistance iron has a tendency to have low flux permeability and to be mechanically brittle and expensive. It is seldom justified in dc machines. The loss is kept to an acceptable value by the use of thin core laminations, 0.017 to 0.025 in thickness.

Another significant loss is the pole-face loss. These losses are included in the core loss.

Unfortunately, there are other losses in the core that may differ widely even on duplicate machines and that do not lend themselves to calculation. These include:

1. Loss due to filing of slots. When the laminations have been assembled, it will be found in some cases that the slots are rough and must be filed to avoid cutting the coil insulation. This burrs the laminations and tends to short circuit the interlaminar resistance.

2. Losses in the solid spider, core end plates, and coil supports from leakage fluxes may be appreciable.

3. Losses due to nonuniform distribution of flux in the rotor core are difficult to anticipate. In calculating core density, it is customary to assume uniform distribution over the core section.

However, flux takes the path of least resistance and crowds behind the teeth until saturation forces it into the less used, longer paths below. As a result of the concentration, the core loss, which is about proportional to the square of the density, is greater than calculated.

Thus, it is not possible to predetermine the total core loss by the use of fundamental formulas. Consequently, core-loss calculations for new designs are usually based on the results from tests on similar machines built under the same conditions.

Such test results are plotted in Fig. 8-54 for machines using ordinary laminations 0.017 in thick and a limited amount of filing. They do not include the pole-face losses, which would increase the values about 30%.

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